on one's last legs是指一个人的腿抽筋，或者腿的机能已达极限，无法再动。引申为“几乎崩溃”（无法再冷静处事）；日暮途穷；黔驴技穷；生命即将结束。

【例句】
The company was on its last legs after the manager retired.
公司在经理退休后到了快解体的地步。

Like a grasshopper at the end of autumn, the enemy is on his last legs.
敌人像秋后的蚂蚱，蹦跶不了几天啦。